1. Field of the Invention
The present invention relates to a projection zoom lens, and more particularly to a projection zoom lens for use in a projection optical system of a projector and also for an image-taking optical system of an image-taking device.
2. Description of the Prior Art
Projectors have been widely used in many applications among which the most important application is compact-sized image-related devices. For example, a projector connected to a portable computer or a desktop computer can satisfy many specific requirements of different environments. A commonly known projector, such as a DLP (Digital Light Processing) projector, uses a digital micro-mirror device (DMD) modulator as a light valve, wherein minute mirror-surface elements (micro-mirrors) are arranged in an array corresponding to pixels and form an image by controlling the angles of the respective mirror-surfaces, and a projection zoom lens enlarges and display the image on a large screen.
Quality of a projected image displayed on a large screen is generally determined by the light valve, the projection zoom lens, and other imaging components thereof. Generally, the projection zoom lens is a compound lens composed of a combination of a plurality of lens elements, at least including a converging lens and a diverging lens, with an attempt to minimize deterioration in resolution, spherical aberration, chromatic aberration, and other type of aberrations.
A conventional projection zoom lens, which is disclosed in U.S. Pat. No. 6,590,716 B2, includes a first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a negative refractive power, and a fourth lens group having a positive refractive power. When the projection zoom lens varies power from a limit wide-angle power toward a limit telephoto power, the first and fourth lens groups are kept stationary and the second and third lens groups undergo proper movement along the optical axis to thereby change the overall focal length. However, this patent only provides the condition between some lens elements of the first and second lens groups, and does not disclose any relationship between the third and other lens groups. Since the movement of the third lens group directly affects the quality of an image projected by the projection zoom lens, it is desired to provide improved modification of the known projection zoom lens in order to realize high quality of image projection.
Hence, it is desired to provide a projection zoom lens of the kind just described above that effectively removes the shortcoming of conventional design and device.